1. Field of the Invention
The present invention relates to a wavefront-measuring interferometer apparatus for carrying out wavefront measurement on a light beam to be measured, and a light beam measurement apparatus and method thereof for carrying out wavefront measurement on a light beam and various types of measurement on a condensed spot of the light beam.
2. Description of the Prior Art
Hitherto, there has been known an apparatus (also known as a beam profiler) that forms a spot image on a CCD image pickup surface or the like of a light beam to be measured, and carries out measurement of the size or shape, or the intensity distribution or barycentric coordinates or the like of the spot image (hereinafter these are referred to collectively as ‘light beam spot characteristic measurement’) (see Japanese Unexamined Patent Publication No. 2004-45327).
Moreover, as an apparatus that carries out wavefront measurement on a light beam, there has been known a wavefront-measuring interferometer apparatus having a Mach-Zehnder interferometer optical system layout as shown in FIG. 8.
With the wavefront-measuring interferometer apparatus shown in FIG. 8, a light beam emitted from a light source unit 101 is divided into two luminous fluxes by a beam splitter 102. One of these two luminous fluxes is converged by a converging lens 103, and is then incident on a pinhole 104 disposed at the convergent point of the converging lens 103. The pinhole 104 is constituted so as to have a diameter smaller than the diffraction limit of the converged luminous flux, so that a wavefront-shaped ideal spherical wave is emitted from the rear of the pinhole 104. This spherical wave is incident on a collimator lens 105 and is thus converted into a plane wave, and is then reflected through a right angle by a mirror 106, before being incident on a beam splitter 107 as reference light.
The other luminous flux divided off by the beam splitter 102 is reflected through a right angle by a mirror 108, and is then converged by a converging lens 109; a pinhole is not disposed at the convergent point of the converging lens 109. The luminous flux transmitted through the converging lens 109 is thus first converged and is then incident on a collimator lens 110 while diverging and is made into parallel light, without wavefront shaping being carried out, and is then incident on the beam splitter 107 as sample light.
The reference light and the sample light are combined at the beam splitter 107, whereby interfering light is obtained, and this interfering light is taken into an image pickup camera 112 via an image-forming lens 111. The wavefront measurement on the light beam is then carried out based on interference fringes picked up by the image pickup camera 112.
The pinhole described above has a function of forming an ideal spherical wave, and is such that the formed spherical wave is emitted to the rear of the pinhole. In contrast with this, there is also known an apparatus having a function of converting part of an incident luminous flux into an ideal spherical wave, and reflecting this spherical wave back in the opposite direction to the direction of incidence (hereinafter referred to as a ‘reflection diffracting part’). Such a reflection diffracting part is also known as a reflection-type pinhole or the like, and ones in which a minute reflection region is formed on a glass substrate, or a minute reflection region is formed on the tip of a needle-shaped member (see Japanese Unexamined Patent Publication No. 2000-97612), ones in which a reflecting surface is disposed immediately behind an ordinary pinhole (see Japanese Unexamined Patent Publication No. S58-60590), and so on are known.
With a conventional Mach-Zehnder wavefront-measuring interferometer apparatus as described above, it is such that the optical elements such as beam splitters and mirrors are disposed symmetrically, and the two luminous fluxes that are made to interfere with one another pass through these optical elements one at a time symmetrically along the respective optical paths; such an interferometer apparatus thus has the characteristic feature that if the optical characteristics of the symmetrically disposed optical elements are made to be equal, then aberration and so on possessed by the respective optical elements will not be prone to having an adverse effect on the measurement results. Mach-Zehnder wavefront-measuring interferometer apparatuses are thus commonly used as highly versatile measurement apparatuses in wavefront measurements on light beams.
However, a Mach-Zehnder wavefront-measuring interferometer apparatus has many optical system components, and there are many places where adjustment must be carried out, and hence there is a problem of adjustment of the optical system being very difficult. Moreover, the optical path of the reference light and the optical path of the sample light must be spatially separated from one another, and hence there is a problem of the apparatus becoming large. Moreover, the constitution is such that the reference light and the sample light pass along separate optical paths, and hence there are problems such as the apparatus being susceptible to vibration, and installation of a phase shifting mechanism being difficult.